It is generally known in the field of aerodynamics, that it is advantageous to form, maintain, and control a smooth, laminar boundary layer flow over an aerodynamic surface, and particularly the leading edge area of an aerodynamic surface, to reduce aerodynamic drag. As described in the article by G. Warwick entitled "Jetstar Smoothes the Way" in Flight International, Sep. 21, 1985, pages 32 to 34, for example, it is possible to provide such laminar flow control by sucking a portion of boundary layer flow through perforations or slots provided in a leading edge structure of an aerodynamic surface such as a wing. In order to achieve this, it is necessary that the outer skin or surface of the leading edge structure is formed by an air permeable, e.g. perforated, skin element. Certain difficulties arise in providing a support structure to support such an air permeable skin element from underneath or from the reverse side thereof.
Published European Patent Application No. 0,665,097 describes a method for manufacturing a leading edge structure of the above mentioned type having a perforated outer skin. More specifically, the leading edge structure comprises a support structure of fiber reinforced synthetic material having channels with a trapezoidal cross-sectional shape extending lengthwise therein, as well as a perforated outer skin that is adhesively bonded onto the support structure so as to form the outer surface of the leading edge. In order to produce such a leading edge structure, the reference discloses a method for forming a support structure for supporting the perforated sheet element that has a front or outer aerodynamic profile-defining surface and a reverse surface. The known method includes steps of supporting the perforated sheet element in a profile defining configuration, forming a mold tool for the support structure at least in part from the reverse surface of the perforated sheet element, forming a lay-up of fiber-reinforcing material together with a plastic or synthetic matrix material in the mold tool, and curing and consolidating the support structure by the application of heat and pressure.
A particular embodiment of the known method according to EP 0,665,097 involves laying into the mold tool a plurality of individual or discrete tool elements in the form of trapezoidal cross-sectional hollow and solid mandrels, which are preferably made of an elastic material such as silicone rubber. When a prepreg material for making the support structure is laid into the mold tool, the mandrels serve to form the prepreg into a corrugated shape. More specifically, the hollow mandrels are first arranged on the perforated sheet material that has been laid into the mold tool, such that small areas of the perforated sheet material remain exposed between adjacent ones of the hollow mandrels. Then the prepreg material is laid onto the hollow mandrels and laminated onto the perforated sheet material in the narrow exposed regions between adjacent mandrels. Next, the solid mandrels are inserted into the gaps formed between the hollow mandrels, and then the inner layer of the support structure is laminated onto the back sides of the solid mandrels and the prepreg supported by the hollow mandrels. This inner layer is then molded and pressed against the other layers by means of a corresponding mold tool, while the hollow interiors of the hollow mandrels are pressurized, whereby all of the prepreg layers are compressed. Heat is applied and the entire composite is cured. Thereafter, the hollow mandrels and the solid mandrels are withdrawn by being pulled lengthwise out of the resulting composite structure. The resulting structure thus comprises the corrugated prepreg layer in the form of corrugations or undulations with hollow corrugation chambers between the outer perforated sheet material and the inner layer.
The above described known method according to European Patent Application 0,665,097 suffers certain problems and disadvantages. Since the mandrels are made of an elastic material such as silicone rubber, they do not exhibit any significant stiffness or form stability, and as a result, it has been found to be quite difficult to properly and accurately lay the mandrels into position during the lay-up process and to remove the mandrels once the structure has been cured. Problems especially arise when the mandrels have a substantial length. In the known method, the molding of the support structure on the backside or reverse side of the perforated sheet element is naturally carried out in a negative mold matching and supporting the outer surface of the perforated sheet. As a result, the lay-up and forming operations are all performed from the backside or inner side of the structure, which leads to restricted accessibility and difficulty in preforming the lay-up procedure, especially if configurations having a tight radius of curvature are to be produced.